DESCRIPTION: This project deals with the mechanisms that account for regulation of ribosomal RNA synthesis. The mammalian rDNA promoter consists of two elements: a core promoter (CPE) and an upstream element (UPE). These interact with at least two transcription factors: UBF and SL1. SL1 binds both UPE and CPE and is required for transcription in vitro. UBF binds the CPE and is necessary for maximum transcriptional activity in vitro. The working hypothesis holds that UBF bends the DNA so as to approximate the SF1 binding sites. Rb blocks activation by UBF and prevents interaction between UBF and SL1. The expression of UBF is regulated by growth stimulation and differentiation, and the activity of UBF is regulated by phosphorylation. This project focuses primarily on the regulation of UBF expression and activity and the role of these processes in regulation of rDNA transcription.